The present invention relates to a method and apparatus for performing seismic reflection surveys from oil tankers or other bulk liquid carrier ships.
2. Description of Prior Art
Marine seismic surveys have ordinarily been made by vessels specifically adapted for the purpose. These vessels have four required items of equipment on board:
a seismic source, or plurality of such sources, capable of generating a large acoustic pulse in the water, a "streamer" containing one or more acoustic detecting arrays, towed behind the vessel at some 15 meters below the sea surface, PA0 A set of digital recording equipment, by which the received seismic signals are recorded on magnetic tape, and displayed for visual assessment, and PA0 A positioning system, by which the vessel may accurately follow a desired course.
Such vessels were normally commissioned to perform surveys in areas known by other means to be generally prospective for hydrocarbon deposits. Since the use of these vessels for surveying was expensive, it was rare for such vessels to be used for reconnaissance in the broadest sense of exploring untested areas to find other generally prospective regions.
As interest grew in the regions beyond the continental shelves, it has become increasingly desirable to perform the broad reconnaissance function from other vessels plying across the continental shelves as part of their normal course.
Accordingly it has been proposed to use oil tankers for this purpose. The proposal is to equip them with a seismic source, a seismic streamer and a set of recording instruments, and to allow them to record continuous seismic traverses along their normal journeys. However, several difficulties existed in the direct transfer of the equipment from a seismic survey vessel to an oil tanker. The first such difficulty arose because of the greater speed of a large tanker: the tow noise generated in a streamer increased markedly wit increased speed through the water. The second difficulty arose in the handling of the streamer: the greater mechanical noise, the greater wake and the greater height of the tanker stern all militated against a short streamer (even for single-channel operation), and this meant an unwelcome amount of additional gear at the stern of the tanker. A third difficulty arose in the disposition of the source or sources: they had to be of a type deemed safe on a tanker (so that explosive or electric sources were undesirable), and the sources were not easy to stream at adequate depth at tanker speeds.
In face of these difficulties, it was proposed to adapt standard seismic techniques to tanker operation by streaming both the streamer and the sources from the side (rather than the stern) of the tanker. However, this also introduced major noise problems and major handling problems.